The present invention relates to powder coating binders containing polyurethanes having (meth)acryloyl groups which are applied onto a substrate, fused with heat and cured by electromagnetic radiation and to a process for preparing the same.
The use of oligo- and polyurethanes as pulverulent binders makes it possible to obtain high quality coatings. While EP-A 410 242 describes the production of such binders in organic solvents, DE-A 19 939 843 describes solvent-free production of pulverulent binders using an elevated fraction by weight of 10 to 95% of a crystalline component which is synthesized from a mono- or diisocyanate and a monohydroxyalkyl (meth)acrylate, wherein the synthesis without solvent is possible only due to the viscosity-reducing action of the molten, crystalline component. DE-A 2 436 186 describes a urethane acrylate which is not stable in storage but is grindable at room temperature.
The disadvantage of the oligo- and polyurethanes described in EP-A 410 242 and DE-A 19 939 843 is poor grindability. The products described in these publications can only be ground without agglomerating at temperatures below 0xc2x0 C. and all subsequent powder handling must proceed at below room temperature as caking of the powder particles is otherwise observed. It is moreover desirable to reduce the proportion of the components synthesized from a mono- or diisocyanate and a monohydroxyalkyl (meth)acrylate as the toxicological profile of low molecular weight acrylates is disadvantageous in a powder application.
In the case of amorphous materials, glassy properties are determined by investigating the material above and below its glass transition temperature. Below its glass transition temperature, grindable products are obtained. However, if an excessively high glass transition temperature is selected, the products only melt at very high temperatures. The desired glass transition temperature of a powder coating is considerably influenced by the ambient temperatures arising during powder coating.
In summer and, in particular, in the vicinity of the ovens required to fuse the powder coatings, temperatures higher than conventional room temperatures of 20 to 25xc2x0 C. must be anticipated. In practice, a sufficiently high glass transition temperature is thus required for a powder coating. These two criteria inversely determine one another, i.e., a desired elevated glass transition temperature results in undesirably high viscosities and vice versa.
In order to achieve the lowest possible fusion temperature, an amorphous powder coating material must accordingly have the lowest possible viscosity and simultaneously exhibit the lowest possible glass transition temperature. Elevated ambient temperatures naturally occur in the powder coating process in summer and/or upon exposure to sunlight, on extended storage or in the vicinity of fusion ovens. 40xc2x0 C. may be considered an upper temperature which is not infrequently observed such that blocking resistance is of industrial significance even at these elevated temperatures.
DE-A 2 436 186 describes the production of a radiation-curable urethane acrylate. This compound comprises the reaction product of 1 mole of trimethylolpropane, 3 moles of toluene diisocyanate and 3 moles of hydroxyethyl methacrylate (See Comparison Example 35) and has a glass transition temperature of 43.7xc2x0 C. and a melt viscosity of 220 Paxc2x7s at 100xc2x0 C. In comparison, Example 9 (which is according to the invention) with the next higher glass transition temperature of 46.1xc2x0 C., and according to the short sprayability test after heat treatment of the powders at 40xc2x0 C. as illustrated in Example 40, illustrates that Example 9, which is according to the invention, is still sprayable after 7.5 hours, while Example 35, which is according to DE-A2 436 186, has already agglutinated after just 2 hours.
The object underlying the invention is thus to provide storage-stable, pulverulent urethane acrylates which are grindable at room temperature, which have no tendency to cake and which may furthermore be produced without solvents and, in comparison with known acrylates, exhibit a particularly low viscosity.
It has now been found that storage-stable powder coatings which are readily grindable at room temperature and which may also be produced without solvents in a melt are obtained if suitable polyols and polyisocyanates are selected and a suitable molecular weight is established. The latter property is achieved by adjusting the feed ratios of the raw materials.
The present invention relates to a binder for a powder coating composition which contains at least one urethane acrylate having a glass transition temperature within the range of from 45 to 80xc2x0 C. which is curable with high energy radiation and which, without the additives conventional in a coating formulation, exhibits a melt viscosity xcex7 at 100xc2x0 C. of less than 1000 Paxc2x7s, provided that the urethane acrylate is not the reaction product of 1 mole of trimethylolpropane, 3 moles of toluene diisocyanate and 3 moles of hydroxyethyl methacrylate.
The present invention also relates to a process for preparing a binder for a powder coating composition from polyurethanes containing (meth)acryloyl groups without using organic solvents which is curable with high energy radiation and which, without the additives conventional in a coating formulation, exhibits a melt viscosity xcex7 at 100xc2x0 C. of less than 1000 Paxc2x7s, by reacting:
A) one or more di- or polyisocyanates or mixtures thereof, optionally with the addition of one or more monoisocyanates;
B) one or more di- or polyfunctional isocyanate-reactive compounds or mixtures thereof; and
C) one or more monofunctional isocyanate-reactive compounds;
wherein 1.) the ratio of NCO groups in A) to the sum of the equivalents of isocyanate-reactive groups in B) and C) is between 0.8 and 1.2 and (meth)acryloyl groups are present either in B) or C) or in both B) and C), such that 2.) the ratio of the equivalents of isocyanate-reactive groups on monofunctional compounds C) to the equivalents of isocyanate-reactive groups on the di or polyfunctional compounds B) is less than 1.2 and 3.) the di or polyfunctional isocyanate-reactive compounds B) have an average functionality of less than 2.5; and 4.) the proportion by weight of soft groups in the di- or polyfunctional isocyanates and in the di- or polyisocyanate-reactive components is less than 25 wt. %, based on the total weight of A), B) and C).
The present invention further relates to the use of the binders according to the invention for powder coatings for coating substrates of wood, metal, plastics, mineral substances and/or precoated substrates made therefrom, or substrates made therefrom which contain any desired combinations of the stated materials. Applications which may in particular be mentioned are in the industrial coating of MDF sheets, preassembled high-grade products which already contain temperature-sensitive sub-assemblies, together with the coating of furniture, coils, everyday articles, automotive bodies and parts attached thereto. The urethane acrylates according to the invention may here also be used in combination with one another or with other binders conventional in powder coating chemistry, for example with polyesters, polyacrylates, polyethers, polyamides, and polycarbonates which may also optionally contain unsaturated groups.